Regardless of the techniques followed or the care taken in fabricating structural components, they still contain flaws which usually take the form of cracks. In pipelines, for example, the flaws may be present in the welds which join pipe sections. The problem associated with flaws or cracks is especially pronounced in aircraft structural materials. The flaws of greatest significance in aircraft structural components are those emanating from the multitude of fastener holes that are drilled in the various components. These components are subjected to rigorous inspections, and specifications place a limit on the maximum length that a flaw may have.
The flaws in aircraft structural components grow as a result of the effects of fatigue or service loading as well as exposure to diverse environmental conditions. The growth of even one flaw beyond a predetermined limit can threaten the integrity of the entire aircraft. A great deal of research work has been directed toward the development of techniques, procedures and instruments to assess structural damage accumulation. The many instruments that have been developed to facilitate damage accumulation tracking are commonly referred to as fatigue gages. These gages have been generally unsuccessful since it has not been possible to relate the gage response to accumulation of structural damage. In U.S. Pat. No. 3,979,949, a fatigue gage is disclosed that falls in this latter category. The gage consists of a rectangular metal strip having a slit therein and a strip of parting material of uniform width attached to the undersurface of the strip and centered on the slit. The gage is attached to a structural member, and changes in the slit length is measured as a function of time. There is no correlation between slit growth and the growth of a defect in the structure. In other words, the method disclosed in the patent does not provide a solution to the problem of predicting the growth of potential preexisting flaws in structural components.
The method currently in use for solving the above-mentioned problem involves the attachement of load sensing elements to an aircraft. The sensing elements count the number of times that a particular component experiences an acceleration that exceeds a specific value. These data are tabulated and used as input to mathematical models which predict flaw growth. This method is inaccurate because of many factors. For example, the loading or acceleration data must be carefully taken from the operational aircraft and accurately entered into a computer memory. Considering the likely chances for error in performing these steps, there is very little confidence in the reliability of these data. Furthermore, the mathematical models used to predict flaw growth from loading data are inaccurate.
It is a principal object of this invention, therefore, to provide a method for predicting damage accumulation in a specific structural component based on the service that it has experienced while eliminating the inaccurate steps of the prior art methods.
Another object of the invention is to provide a method for accurately tracking the growth of potential preexisting structural flaws.
A further object of the invention is to provide a method for assessing flaw growth potential in structural components.
Still another object of the invention is to provide a crack growth gage having a flaw that may be used to predict the residual life of the structural component to which it is attached.
A still further object of this invention is to provide a preflawed gage that functions as an on-board analog computer, incorporating the effects of both fatigue or service loading or environment on a real time basis, in the prediction of damage accumulation in a structural component.